DESCRIPTION

This library implements type-checked dynamic hash tables. The hash
table entries can be arbitrary structures. Usually they consist of key
and value fields.

lh_<type>_new() creates a new LHASH_OF(<type> structure to store
arbitrary data entries, and provides the 'hash' and 'compare'
callbacks to be used in organising the table's entries. The hash
callback takes a pointer to a table entry as its argument and returns
an unsigned long hash value for its key field. The hash value is
normally truncated to a power of 2, so make sure that your hash
function returns well mixed low order bits. The compare callback
takes two arguments (pointers to two hash table entries), and returns
0 if their keys are equal, non-zero otherwise. If your hash table
will contain items of some particular type and the hash and
compare callbacks hash/compare these types, then the
DECLARE_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN macros can be
used to create callback wrappers of the prototypes required by
lh_<type>_new(). These provide per-variable casts before calling the
type-specific callbacks written by the application author. These
macros, as well as those used for the ``doall'' callbacks, are defined
as;

lh_<type>_free() frees the LHASH_OF(<type> structure
table. Allocated hash table entries will not be freed; consider
using lh_<type>_doall() to deallocate any remaining entries in the
hash table (see below).

lh_<type>_insert() inserts the structure pointed to by data into
table. If there already is an entry with the same key, the old
value is replaced. Note that lh_<type>_insert() stores pointers, the
data are not copied.

lh_<type>_delete() deletes an entry from table.

lh_<type>_retrieve() looks up an entry in table. Normally, data
is a structure with the key field(s) set; the function will return a
pointer to a fully populated structure.

lh_<type>_doall() will, for every entry in the hash table, call
func with the data item as its parameter. For lh_<type>_doall()
and lh_<type>_doall_arg(), function pointer casting should be avoided
in the callbacks (see NOTE) - instead use the declare/implement
macros to create type-checked wrappers that cast variables prior to
calling your type-specific callbacks. An example of this is
illustrated here where the callback is used to cleanup resources for
items in the hash table prior to the hashtable itself being
deallocated:

/* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
void STUFF_cleanup_doall(STUFF *a);
/* Implement a prototype-compatible wrapper for "STUFF_cleanup" */
IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF)
/* ... then later in the code ... */
/* So to run "STUFF_cleanup" against all items in a hash table ... */
lh_STUFF_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
/* Then the hash table itself can be deallocated */
lh_STUFF_free(hashtable);

When doing this, be careful if you delete entries from the hash table
in your callbacks: the table may decrease in size, moving the item
that you are currently on down lower in the hash table - this could
cause some entries to be skipped during the iteration. The second
best solution to this problem is to set hash->down_load=0 before
you start (which will stop the hash table ever decreasing in size).
The best solution is probably to avoid deleting items from the hash
table inside a ``doall'' callback!

lh_<type>_doall_arg() is the same as lh_<type>_doall() except that
func will be called with arg as the second argument and func
should be of type LHASH_DOALL_ARG_FN_TYPE (a callback prototype
that is passed both the table entry and an extra argument). As with
lh_doall(), you can instead choose to declare your callback with a
prototype matching the types you are dealing with and use the
declare/implement macros to create compatible wrappers that cast
variables before calling your type-specific callbacks. An example of
this is demonstrated here (printing all hash table entries to a BIO
that is provided by the caller):

NOTE

The various LHASH macros and callback types exist to make it possible
to write type-checked code without resorting to function-prototype
casting - an evil that makes application code much harder to
audit/verify and also opens the window of opportunity for stack
corruption and other hard-to-find bugs. It also, apparently, violates
ANSI-C.

The LHASH code regards table entries as constant data. As such, it
internally represents lh_insert()'d items with a ``const void *''
pointer type. This is why callbacks such as those used by lh_doall()
and lh_doall_arg() declare their prototypes with ``const'', even for the
parameters that pass back the table items' data pointers - for
consistency, user-provided data is ``const'' at all times as far as the
LHASH code is concerned. However, as callers are themselves providing
these pointers, they can choose whether they too should be treating
all such parameters as constant.

As an example, a hash table may be maintained by code that, for
reasons of encapsulation, has only ``const'' access to the data being
indexed in the hash table (ie. it is returned as ``const'' from
elsewhere in their code) - in this case the LHASH prototypes are
appropriate as-is. Conversely, if the caller is responsible for the
life-time of the data in question, then they may well wish to make
modifications to table item passed back in the lh_doall() or
lh_doall_arg() callbacks (see the ``STUFF_cleanup'' example above). If
so, the caller can either cast the ``const'' away (if they're providing
the raw callbacks themselves) or use the macros to declare/implement
the wrapper functions without ``const'' types.

Callers that only have ``const'' access to data they're indexing in a
table, yet declare callbacks without constant types (or cast the
``const'' away themselves), are therefore creating their own risks/bugs
without being encouraged to do so by the API. On a related note,
those auditing code should pay special attention to any instances of
DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types
without any ``const'' qualifiers.

BUGS

lh_<type>_insert() returns NULL both for success and error.

INTERNALS

The following description is based on the SSLeay documentation:

The lhash library implements a hash table described in the
Communications of the ACM in 1991. What makes this hash table
different is that as the table fills, the hash table is increased (or
decreased) in size via OPENSSL_realloc(). When a 'resize' is done, instead of
all hashes being redistributed over twice as many 'buckets', one
bucket is split. So when an 'expand' is done, there is only a minimal
cost to redistribute some values. Subsequent inserts will cause more
single 'bucket' redistributions but there will never be a sudden large
cost due to redistributing all the 'buckets'.

The state for a particular hash table is kept in the LHASH structure.
The decision to increase or decrease the hash table size is made
depending on the 'load' of the hash table. The load is the number of
items in the hash table divided by the size of the hash table. The
default values are as follows. If (hash->up_load < load) =>
expand. if (hash->down_load > load) => contract. The
up_load has a default value of 1 and down_load has a default value
of 2. These numbers can be modified by the application by just
playing with the up_load and down_load variables. The 'load' is
kept in a form which is multiplied by 256. So
hash->up_load=8*256; will cause a load of 8 to be set.

If you are interested in performance the field to watch is
num_comp_calls. The hash library keeps track of the 'hash' value for
each item so when a lookup is done, the 'hashes' are compared, if
there is a match, then a full compare is done, and
hash->num_comp_calls is incremented. If num_comp_calls is not equal
to num_delete plus num_retrieve it means that your hash function is
generating hashes that are the same for different values. It is
probably worth changing your hash function if this is the case because
even if your hash table has 10 items in a 'bucket', it can be searched
with 10 unsigned long compares and 10 linked list traverses. This
will be much less expensive that 10 calls to your compare function.

lh_strhash() is a demo string hashing function:

unsigned long lh_strhash(const char *c);

Since the LHASH routines would normally be passed structures, this
routine would not normally be passed to lh_<type>_new(), rather it would be
used in the function passed to lh_<type>_new().

SEE ALSO

HISTORY

The lhash library is available in all versions of SSLeay and OpenSSL.
lh_error() was added in SSLeay 0.9.1b.

This manpage is derived from the SSLeay documentation.

In OpenSSL 0.9.7, all lhash functions that were passed function pointers
were changed for better type safety, and the function types LHASH_COMP_FN_TYPE,
LHASH_HASH_FN_TYPE, LHASH_DOALL_FN_TYPE and LHASH_DOALL_ARG_FN_TYPE
became available.

In OpenSSL 1.0.0, the lhash interface was revamped for even better
type checking.